1. Field of the Invention
This invention relates in general to materials comprised of shape memory alloys (also called SMA). The invention also relates to the fabrication of thin film SMA of the type used in devices employed in various fields, such as aerospace, medicine, instrumentation and consumer products.
2. Description of the Related Art
Shape memory alloys have been employed in various devices and products, as for example actuators, valves, switches, latches and the like. Miniature devices made of SMA in thin film form have been advantageously employed in Microelectronic mechanical systems (called MEMS). The basic procedures for fabricating SMA in thin film form are disclosed in the Busch et. al. U.S. Pat. No. 5,061,914 issued Oct. 29, 1991 and which is assigned to the assignee of the present invention.
As is well known, an SMA thin film or other form undergoes a crystalline phase change or transformation from martensite to austenite when heated through the material""s phase-change transition temperature. When below that temperature in a xe2x80x9ccold statexe2x80x9d the material can be plastically deformed responsive to stress. When the SMA is heated through the transition temperature, it forcefully reverts to its xe2x80x9cmemory shapexe2x80x9d while exerting considerable force.
A common SMA material with usable shape memory alloy properties is comprised of TiNi. These TiNi SMA materials have good thermo-mechanical properties, but their phase-change transition temperatures are limited to less than 100xc2x0 C. Binary TiNi has a transition temperature with an upper limit of about 90xc2x0 C. SMA materials of TiNiHf as well as TiNiPd have been shown to have transition temperatures up to 300xc2x0 C., but these have not been commercially acceptable because they are generally brittle and difficult to fabricate into devices. It is desirable to have SMA material with transition temperatures higher than 100xc2x0 C. while retaining the desired ductility and shape memory effect. Many applications require higher transition temperatures, but no conventional SMA material has been found to be satisfactory for these purposes.
The shape memory effect of an SMA material which is desirable for various applications, such as for actuators used in aerospace, medicine, the military and consumer products, derives from an energetic thermally driven crystalline phase change. The two phases, termed austenite and martensite, have radically different mechanical properties, and a very large amount of mechanical work can be recovered during the transformation. The most widely used SMA, namely TiNi (also called Nitinol), is an equi-atomic alloy of titanium and nickel. The TiNi phase transformation temperature depends critically upon the stoichiometry: increasing the atomic percentage of Ni lowers the transformation temperature, while increasing the Ti atomic percentage raises that temperature to a maximum of about 100xc2x0 C. Many of the applications contemplated for the use of an SMA material are in situations where the ambient temperature exceeds 100xc2x0 C. Previous research has demonstrated transition temperatures in excess of 100xc2x0 C. for ternary TiNi-based alloys containing hafnium (replacing titanium) and palladium (replacing nickel), but generally the TiNiHF and TiNiPd alloys produced in experimental quantities exhibited large hysteresis and brittleness. Practical alloys that overcome these deficiencies and that have a phase-change transition temperature above 100xc2x0 C. will expand its use potential in a variety of markets.
The need has therefore been recognized for a method of fabrication, and materials made by the method, comprised of high transition temperature SMA materials which obviate the foregoing and other limitations and disadvantages of prior art SMA. Despite the various SMA materials in the prior art, there has heretofore not been provided a suitable and attractive solution to these problems.
It is a general object of the invention to provide a method of fabricating shape memory alloys into thin films, and SMA materials made by the method.
Another object is to provide a method of fabrication thin film shape memory alloys, and alloys made by the method, that have optimal thermo-electrical properties, particularly with higher phase-change transition temperatures.
A further object is to provide a method of fabricating thin film shape memory alloys, and alloys made by the method, that have phase-change transition temperatures well above 100xc2x0 C.